Note: Descriptions are shown in the official language in which they were submitted.
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A method and a fibre distributor for air-laying fibres
The invention relates to a method and a fibre distributor for
air-laying fibres on an endless, air pervious forming wire in
a plant for producing non-woven webs for e.g.,
- absorbent core material for feminine hygiene articles,
- incontinence articles,
- diapers,
- table top napkins,
- hospital products such as bed protection sheets,
- wipes, and
- towels.
The fluff, which is usually employed for products of this
nature, is fluff of relatively short cellulose fibres, of
relatively long synthetic fibres, or of a blend of such
fibres. Other materials like e.g. SAP (Super Absorbent Powder)
can be mixed into the fluff.
The fibre distributor comprises a forming head, which is
placed above the forming wire and has a perforated bottom and
at least one row of rotateable wings situated at a distance
above said bottom.
The fibres supplied to the forming head are, during
production, swept along by rows of wings in an air stream in
order to be evenly distributed over the bottom. A suction
aggregate, placed below the forming wire, simultaneously
generates a second air stream through the openings in the
perforated bottom of the fibre distributor and through the
forming wire, whereby fibres entrained in the air stream are
successively deposited in a layer on the upper part of the
forming wire.
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Such a fibre distributor is known from the applicant's US
patent No. 5.527.171, which is included in the present
application as reference.
However, the relatively long synthetic fibres are especially
inclined to form nits with other similar or differing types of
fibres while being swept along in the air stream by the rows
of wings in the fibre distributor.
Nits are small tangled bundles of fibres, which can be quite
hard and difficult to open even when they are recirculated
back to e.g. a hammer mill. The nits contribute neither to the
volume, the quality, nor the strength of the non-woven webs.
The generation of nits during production of the webs should
therefore be kept to as low a level as possible.
However, in practice, up to about 25% of the supplied
synthetic fibres can be reduced to useless nits. The fibres
lost in this way need to be replaced by the same quantity of
good fibres. Since the synthetic fibres are especially costly
the finished fibre product will therefore also be costly.
Previous experiments carried out to find a solution to this
problem had shown that the contents of fibres per unit of area
of the finished non-woven web, which are reduced to nits,
grows as a function of the velocity of the air stream, in
which the fibres are swept along by means of the rows of
wings.
The experiments also revealed that the capacity of the fibre
distributor and thereby of the entire plant for producing the
non-woven webs also grows as a function of said velocity.
Therefore, since the financial investments in such plants are
very high, the plants should be operated with as high a
velocity of the air stream along the rows of wings as
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possible. The velocity should, on the other hand, also be very
low in order to keep the contents of fibres per unit of area
of the finished non-woven web, which are reduced to nits, to
as low a level as possible.
Therefore it has, in practice, been necessary to reach a
compromise where a plant is operated with relatively small
velocities of the air stream along the rows of wings,
resulting in that the capacity of the plant is far from being
fully utilized.
It is an object of the invention to remedy the above-mentioned
drawbacks of the known technique.
In a first aspect of the invention is provided a fibre
distributor of the kind named in the preamble, which has a
higher production capacity than hitherto known.
In a second aspect of the invention is provided a fibre
distributor of the kind named in the preamble, which during
operation generates fewer nits than hitherto known.
In a third aspect of the invention is provided a fibre
distributor of the kind named in the preamble, which during
operation generates fewer nits than hitherto known and at a
higher rate of production than hitherto known.
In a fourth aspect of the invention is provided a fibre
distributor of the kind named in the preamble, which is
arranged to, during operation, regulate the process in such a
manner that a minimum of nits are generated.
In a fifth aspect of the invention is provided a method of the
kind named in the preamble by which it is possible to obtain a
higher production capacity of a fibre distributor than
hitherto known.
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In a sixth aspect of the invention is provided a method of the
kind named in the preamble by which it is possible to operate
a fibre distributor such that lesser nits than hitherto known
are generated.
In a seventh aspect of the invention is provided a method of
the kind named in the preamble by which lesser nits than
hitherto known are generated at a higher rate of production
than hitherto known.
In an eighth aspect of the invention is provided a method of
the kind named in the preamble by which the process can be
regulated in such a manner that a minimum of nits are
generated.
The fibre distributor is, according to the invention, arranged
for regulating the speed of rotation of the wings into an
interval of speeds around an optimal speed where the fibres
inclination to form nits changes from being lesser to being
larger when increasing the rotation speed of the wings.
This interval of speeds is much higher than the skilled person
in the art has so far believed could lead to a positive
result, since experiments carried out to solve the problem of
fibre loss due to the formation of nits showed that the loss
of fibres increased when the speed of rotation of the wings
grows.
At said optimal speed the production rate at the same time
also is increased, whereby the capacity of the fibre
distributor and thereby of the entire plant is advantageously
utilized much better than hitherto known.
Said optimal speed of rotation of the wings can in practise
vary depending on the prevailing conditions of the fibre
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structure and fibre composition and also of the production
parameters of the fibre distributor.
The fibre distributor could therefore be arranged to regulate
5 the speed of rotation of the wings into an interval of speeds
of rotation, which could include speeds, which by certain
conditions are not completely optimal.
This interval can be around an average speed of rotation and
the regulation can be carried out in agreement only with the
composition of the fibres and the arrangement of the actual
fibre distributor. That means that the speed of rotation of
the wings doesn't need to be optimal all the time, since the
optimal speed can vary as explained above.
The speed of rotation of the wings can, in another embodiment
according to the invention, be optimal all the time by
continually regulating the speed to the speed, which at a
given moment is optimal. Thereby it advantageously is obtained
that the loss of fibres due to the formation of nits, is as
low as possible.
This embodiment comprises the steps of detecting the amount of
nits per unit of area of the fibre layer on the forming wire
or in the resulting non-woven web; sending signals
representing the results of the detecting as input to a
computer; using this input to calculate, by means of a program
in the computer, a value which represents an optimal rotation
speed where the number of nits at a given moment are small or
smallest; and sending signals representing this value as
output from the computer to order the wings to rotate with
rotating speeds represented by said value.
The fibre distributor according to the invention has in this
way been self-regulating and will therefore, during operation,
automatically produce non-woven webs with a minimum of loss of
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fibres and at the same time also with an extremely high rate
of production.
It is significant that the loss of fibres due to the formation
of nits is low in order to save expenses by reducing the
amount of material employed to form the non-woven webs. The
loss is of course lowest at the optimal speed of rotation of
the wings.
However, if the profit gained from operating the fibre
distributor at a higher capacity than the capacity at the
optimal rotating speed of the wings is about the same as the
expenses due to the loss of fibres in form of nits, said
profit can be used to control the process instead of the
smallest contents of nits per unit of area of the finished
web.
The invention will be explained in greater detail below where
further advantageous properties and only exemplary embodiments
are'described with reference to the drawings, where
Fig. 1 shows schematically, seen from the side, a fibre
distributor according to the invention for air-laying fibres
on an endless wire by means of a forming head having a
perforated bottom and rotateable wings for sweeping the fibres
along said bottom during operation,
Fig. 2 shows the same, seen from above,
Fig. 3 shows in an enlarged scale a fragment of a non-woven
web,
Fig. 4 are graphs showing the number of nits per unit of area
of a non-woven web and also the output per wing per unit of
time as a function of the velocity at which the fibres are
swept along the perforated bottom of the forming head, and
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Fig. 5 is a block diagram of a controlling system for the air-
laying process carried out by means of the fibre distributor
shown in fig. 1 and 2.
The following detailed description is based on the assumption
that the fibres consist of a blend of relatively short
cellulose fibres and relatively long synthetic fibres.
The fibre distributor 1 comprises a forming head 2 with an
inlet 3 for the cellulose fibres 4 and another inlet 5 for the
synthetic fibres 6. These inlets 3,5 admit the respective
fibres 4,6 to enter the forming head in air streams in the
direction of the arrows shown.
The forming head has a perforated bottom 7 with openings 8.
Below the bottom is arranged an endless, air pervious forming
wire 9 running, during production, over rolls 10 in the
direction shown by the arrow. Only part of said forming wire
is shown in fig. 1 and 2.
Below the forming wire is placed a suction box 11. An
evacuating pump 12 serves for producing a negative pressure in
the suction box via an air duct 13.
At a distance above the perforated bottom are, in this
example, mounted five rows 14, each having three rotateable
wings 15.
The wings are, during operation of the fibre distributor,
rotated with such a speed of rotation that the supplied fibres
are swept along the rows of wings in a first air stream, which
is generated by the rotating wings, thereby distributing the
fibres all over the bottom as illustrated in fig. 2 with the
arrows.
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The fibres in the first air stream are, by means of the
suction box and the evacuating pump, successively sucked down
through the openings 8 in the perforated bottom 7 entrained in
the generated second air stream, thereby depositing the fibres
in a layer 16 on the upper part 17 of the forming wire.
The forming wire transports this layer into the direction of
the arrow for being further treated in the subsequent parts of
the associated plant (not shown) in such a way that the
desired non-woven web is formed.
The synthetic fibres 6 are usually supplied as staple fibres,
while the cellulose fibres 4 are supplied in the form of a
fibre roll (not shown), which is defibrated to fluff by a
hammer mill (not shown).
Fig. 3 illustrates schematically in a larger scale a fragment
of a non-woven web 18, which contains synthetic fibres 6 and
cellulose fibres 4. As it appears from the figure are the
cellulose fibres formed with protuberances 19. The non-woven
web also contains cellulose nits 20 consisting alone of
cellulose fibres and compound nits 21 consisting of both
cellulose fibres 4 and synthetics fibres 6.
Nits are small tangled fibre bundles, which decrease the
quality of the non-woven web. Another disadvantage is that the
nits exist in a very compact form. It is therefore necessary
to increase the supply of fibres by a quantity of fibres
corresponding approximately to the mass of fibres tied up in
the nits, thereby increasing the cost of producing the web.
The fibre roll contains some nits already from the start.
During the defibration process some of these fibres are opened
and formed to good fibres. But at the same time some other
fibres are formed to nits in a normally larger scale than the
quantity of fibres tied up in nits, which are opened. The
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percentage of nits in the defibrated fluff also increases with
the defibrating speed, say from about 1% to about 1,4%.
On the way from the hammer mill to the fluff layer 16 on the
forming wire 17 more cellulose nits 20 can be generated in the
fluff. The cellulose fibres 20 act as a kind of nucleus for
forming the compound nits 21. The protuberances 19 play an
important part in this connection in that they are capable of
catching the synthetic fibres.
The nits show a tendency to grow during the air-laying
process. Having attained a certain size, the nits then tend to
split up into two or more nits, which then acts as new
nucleuses for forming still more nits.
Experience has shown that the contents of fibres per unit of
area of the finished non-woven web tied up in nits, grows with
the speed of the first air stream similar to the process in
the hammer mill where the percentage of nits also grows with
the defibration in feed speed.
When a non-woven web of good quality that means with a low
contents of nits is desired, the production plant therefore
needs to be operated at a low production speed, whereby the
expenses to the fibres also will be low.
However, the total production costs depends also on the
normally rather large investments in the entire plant. For
paying a sufficient interest on these large investments the
plant needs to be operated at a high production speed.
In practise such plants therefore are operated at production
speeds, where the produced webs have a relatively poor quality
and the entire plant is operated with a relatively low
capacity, resulting in relatively high production costs of the
webs owing to the large consumption of fibres used in the
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production and the low interest paid on the investment in the
plant.
The producer of the webs is in this way reduced to come to a
5 compromise where the quality and the price of the product and
also the utilisation of the potential capacity of the plant
are very far from being optimal.
This unhappy situation is overcomed by means of the present
10 invention, which will be explained more definitively in the
following with reference to fig. 4, which illustrates an
exemplary embodiment of the invention
The figure shows with full line the number of nits pr. m2
(n/m2) in the finished web as function of the velocity v
(m/sec) of the first air stream, which is the air stream
generated by the wings for sweeping the fibres along the
perforated bottom of the forming head. Only nits having a
sectional size larger than 1 mm2 are counted in this example.
With dotted line is in the same figure also shown the output
per wing per hour (kg/w/hour) of the plant as an approximately
linear function of said velocity v (m/sec).
The web produced in the example had a weight of 0,120 kg/m2
and was composed of 80% cellulose fibres and 20% synthetic
fibres. The average length of the cellulose fibres was about 2
mm, while the average length of the synthetic fibres was about
6 mm.
The wings in a row were, as illustrated with the arrows in
fig. 2, rotated in the same direction while the wings in two
adjacent rows were rotated in opposite directions, whereby the
fibres were swept along the perforated bottom and evenly
dispersed over the area of this.
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The perforated bottom was of the type described in the
applicant's patent application WO 9.9/54537, "a sifting net for
a fibre distributor". The mesh size of the net was 4.
The above-mentioned specifications are common for non-woven
webs used for e.g. incontinence articles and also for existing
production plants. The normal velocity of the first air stream
used for producing said non-woven web was 3 m/sec. At this
velocity the number of nits pr. m was found to be 500 n/m with
an output per wing per hour of 12 kg/w/h.
These results are very unsatisfactory. The quality of the
product is poor and a rather high rate of fibres is used for
manufacturing the webs. This fact in combination with the fact
that the gained output also is rather low results furthermore
in high production costs.
The person skilled in the art, which is trying to obtain a
better product, that is to say a product with a lower contents
of nits, will by means of experiments find out that lowering
the velocity v from the normal 3 m/sec to e.g. 1,0 m/sec would
result in that the nits contents in the web advantageously was
decreased from 500 n/m2 to a level of only 67 n/m2, but also in
that this improvement was at the expense of an output as
little as 1.0 kg/w/h.
Owing to the resulting high production cost and the demand for
being able to supply the customers with a sufficient quantity
of products the person skilled in the art soon would realize
that lowering the velocity of the first air stream in this way
could not lead to a useful solution of the problem, that a low
contents of nits and a high production rate are wanted at the
same time. Having arrived to this conclusion he will stop
further experiments with lowering the velocity of the first
air stream.
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When the person skilled in the art, on the other hand, would
try to increase the output for lowering the production costs
in this way he soon would find out that the gained improvement
of output was at the expense of an unacceptable increasing of
the number of nits per m2, whereby he also in this case would
stop further experiments.
Said skilled person would therefore never get the idea to
carry out experiments with velocities, which are far away from
the normal used velocities, since he, by carrying out his
above mentioned experiments, had learned that changing the
velocity up or down from the normal velocity, where the best
compromise between the production parameters were found, does
not imply any improvements.
According to the present invention this technical prejudice is
overcome by operating the wings with such a high speed of
rotation that velocities of the first air stream far away from
the normal used velocities are generated.
In a preferred embodiment according to the invention are the
wings rotated with such a rotation speed that the velocity of
the first air stream is between 9 m/sec and 16 m/sec and
especially between 11 m/sec and 14 m/sec.
In another embodiment according to the invention are the wings
rotated with such a rotation speed that the velocity of the
first air stream is between 5 m/sec and 26 m/sec,
preferentially between 8 m/sec and 17 m/sec, and especially
between 10 m/sec and 15 m/sec.
As it appears from fig. 4 the number of nits per m2 non-woven
web was decreased from 500 n/m2 at a velocity of 3,0 m/sec to
only 117 n/m2 at a velocity v of 12,7 m/sec, while the output
at the same time was raised to 60 kg/w/h.
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By using the technique according to the invention was
surprisingly obtained that the contents of nits in the
finished non-woven web was only about 24% of the normal while
the output was about five times the normal.
Fig. 4 also shows that the contents of nits in the finished
non-woven web increases from 67 n/ma at a velocity of 1,0 m/sec
up to a maximum of 583 n/m2 at a velocity of 4,4 m/sec. After
that the nits contents decreases to a minimum of 60 n/m2 at a
velocity of 12,7 m/sec for thereafter to increase again.
In this case is the velocity of 12,7 m/sec such an optimal
velocity where the optimal lowest nits contents is obtained at
the same time as a very large increase of the output has taken
place. For other plants and webs with other fibre compositions
there is also an optimal velocity, which could be the same as
in this example or have another value than in this example.
In a preferred embodiment of the invention, the mutual
distance between each of two neighbouring rows of wings is the
mutual distance between two wings in a row plus between 50 mm
and 135 mm, preferably between 75 mm and 105 mm.
Production parameters such as the composition and the
structure of the fibres can possibly vary during the
production of the non-woven web whereby the optimal velocity
concurrently will vary. The fibre distributor therefore is
equipped with a regulator for regulating the velocity of the
first air stream in an interval around an average optimal
velocity.
Said interval has according to the invention a size between
0.5 and 1.5, preferentially between 0.75 and 1.25 and
especially between 0.9 and 1.1 times the rotation speed of the
wings generating said average optimal velocity.
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The regulating of the velocity of the first air stream can
advantageously be performed automatically by means of a
controlling system according to the invention, which
schematically is illustrated by means of the block diagram
shown in fig. 5.
This system comprises a detector 22, which is connected to a
computer 23, which again is connected to an actuator 24 for
rotating the wings and also to actuators for driving other
functions of the plant, which possibly should vary when
varying the velocity of the first air stream.
In fig. 5 only actuators 25, 26, 27, 28 and 29 are shown for
driving the functions of feeding the roll of cellulose fibre
to the hammer mill, feeding the cellulose fibre from the
hammer mill to the forming head, feeding e.g. synthetic fibres
to the forming head, driving the forming wire, and generating
the other air stream, but other actuators (not shown) for
driving other functions can also be related to the system.
The detector is, as seen in fig. 1, placed downstream the
forming head and above the fluff on the forming wire and it
can be of any suitable type, for example a digital photo
detector, a laser detector or an ultrasound detector.
The detector is arranged for counting the number of nits per
unit of area of fluff on the wire or non-woven web or the
number of nits per unit of area of fluff on the wire or non-
woven web and also the size of each of these nits.
The result of the detecting is constantly sent as input to the
computer, which also receives input (not shown) for the
simultaneous supplying of the different fibres to the forming
head.
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The computer is loaded with a program, which based on this
information is adapted to calculate the contents of fibres per
m2 of fluff on the wire or of the finished non-woven web.
5 This contents is, as previously explained, a function of the
velocity of the first air stream, which in the example is
illustrated with the curve in full line shown in fig. 4.
The program of the computer is also adapted to calculate the
10 differential quotient for each point of this curve and to
continuously regulate the actuator 24 until the differential
quotient is zero in a point of the curve.
As the operator of the plant now knows the invention and
15 thereby also the interval where the optimal velocity of the
first air stream is expected to be found, the plant will in
practice be started with a velocity in this interval, after
which the velocity continuously is regulated to the optimal
velocity which is precisely the point of the curve where the
differential quotient in a given moment is zero.
The computer of the controlling system for the air-laying
process has, in a preferred embodiment of the invention, a
memory for saving the relevant data, which are obtained during
the production of a specific web. By using this data the plant
easily and quickly can be started up next time the same web is
going to be produced.
Since the differential quotient is zero in both a maximum and
a minimum point of the nits contents curve the program is in
an embodiment of the invention adapted to reject the maximum
zero point for thereby only regulating the velocity of the
first air stream into the minimum zero point, which also is
the optimal point of the curve.
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For performing this is the computer loaded with values for the
velocities of the first air stream used by the known technique
and the program of the computer is adapted to reject a zero
point, which is higher than such a velocity for a given
product.
In this way is it possible to start the production at any
point of the curve.
The computer is also adapted to regulate other actuators, e.g.
the actuators 25, 26, 27, 28 and 29 in correspondence with the
regulating of the velocity of the first air stream.
According to the invention is the fibre distributor in the
above-described way self-regulating and will therefore, during
operation, automatically produce non-woven webs with a minimum
of fibre loss in form of nits and at the same time also with
an extremely high rate of production. The plant is also very
easy to start up and operate.
